Hydrothermal synthesis and characterization of nanostructured tungsten oxide and molybdenum oxide

Abstract

Self-assembled 1-D $\textit{h}-WO_3$ nanowire bundles and urchin-like structures were successfully synthesized through a hydrothermal process. A pure hexagonal phase crystalline $WO_3$ hierarchical nanostructure was confirmed by XRD and TEM analyses. SEM and TEM images revealed self-assembled nanowire bundles and nanorods that formed urchin-like structures. The shapes of the $\textit{h}-WO_3$ nanowire bundles and urchin-like nanostructures could be manipulated by application of $Na^+$ and $NH_4^+$ based EDTA salt solutions in the presence of $Na_2SO_4$. In addition, a particular amount of sodium ions and the ratio of $NH_4^+$ / $Na^+$ ions in the reaction medium plays a unique role even though presence of ammonium ions is required for producing the morphology of urchin-like structure of $WO_3$ nanocrystals. This versatile method provides a straightforward and efficient means of obtaining $WO_3$ nanostructures having unique morphologies. The characteristic properties of the nanowire bundles were considerably enhanced compared to those of the urchin-like structures, because of their highly ordered self-assembled structures. $MoO_3$ Nanobelts, $MoO_2$ Hollow spheres, $MoO_3$ Nanoplatelets and Self-assembled $MoO_2$ Nanoplatelets samples were successfully synthesized through hydrothermal reaction of $Na_2MoO_4$ or $(NH_4)_6Mo_7O_{24}\cdot4H_2O$, HCl and with/without $NH_4^+$ based EDTA salt solutions and $Na_2SO_4$. Unlike tungsten oxide nanoparticles synthesis, $MoO_3$ Nanobelts and $MoO_2$ Hollow spheres could be synthesized when HCl and water solvent were used. A pure phase crystalline molybdenum oxide nanoparticles were confirmed by XRD and TEM analyses. SEM and TEM images revealed nanoplatelets that formed Self-assembled $MoO_2$ Nanoplatelets. The morphologies of the molybdenum oxide nanoparticles could be manipulated by application of $NH_4^+$ based EDTA salt solutions in the presence of $Na_2SO_4$. In addition, unlike tungsten oxide $MoO_3$ Nanobelts and $...